1. Field of the Invention
The subject invention pertains to solid isocyanate terminated prepolymers useful in preparation of one-component non-cellular polyurethane compositions. More particularly, the solid isocyanate derivatives are isocyanate-terminated compounds capable of forming thermosetting polyurethane compositions which may readily be prepared in powdered form without the necessity of grinding.
2. Background of the Invention
One component adhesives are useful commercial compositions. Such adhesives have long pot life as compared to two component adhesives which, in addition to the necessity of thorough mixing, must be used immediately after preparation. In order to have long pot lives, U.S. Pat. Nos. 3,933,759 and 3,963,710 teach the use of blocked isocyanates in the form of fine powders. Upon heating, the blocking agent "unblocks", liberating isocyanate groups which are then free to react. Addition of blocking agent requires additional processing steps, is wasteful of the blocking agent, and moreover produces products in which the presence of the blocking agent may decrease physical properties.
U.S. Pat. Nos. 4,683,279 and 2,969,386 disclose preparation of urethane linked toluene diisocyanate dimers by reacting toluene diisocyanate with a low molecular weight diol. In U.S. Pat. No. 2,969,386, the reaction occurs neat, with an excess of isocyanate, at a temperature above the melting point of the reaction mixture but low enough to avoid side reactions, and the product is then washed repeatedly with solvent to remove unreacted isocyanate. In U.S. Pat. No. 4,683,279, the diol is an unsymmetrical glycol such that the product is either fluid at room temperature or slightly above. The use of a solvent in which both the reactants and products are soluble, such as acetone, dioxane, tetrahydrofuran, or acrylonitrile is optional. The solvent is then removed by stripping or distillation at which time excess toluene diisocyanate is also removed.
U.S. Pat. No. 3,285,951 discloses the formation of solid prepolymers by reacting toluene diisocyanate with a carefully selected mixture of 2,3-butanediol isomers having a mesoisomer content of 40 percent plus two times the difference of 100 and the weight percent of the 2,4-toluene diisocyanate isomer present in the toluene diisocyanate mixture. It is stated that unless this specific glycol composition is utilized, a doughy past is obtained rather than a solid product.
U.S. Pat. No. 5,043,472 teaches the preparation of solid isocyanate terminated prepolymers by reaction of organic isocyanates with 2-methyl-1,3-propanediol. The reaction takes place neat, and the solid reaction product must be ground into fine particles if use in particulate form is desired. The patent indicates that solids prepared from other diols such as ethylene glycol, 1,4-butanediol, and propylene glycol cannot be ground into particles and are thus unsuitable for such applications.
U.S. Pat. Nos. 3,787,525; 3,817,886; and 3,917,741 teach the preparation of thermoplastic polyurethane powders by interfacially reacting a diisocyanate and difunctional compound in an inert solvent in which the isocyanate is insoluble and present in the form of an emulsion to which the difunctional compound is added. The products are relatively high molecular weight polymers which can be melted to form hot melt adhesives. Such thermoplastics adhere by cohesive properties and gap filling properties rather than through potential reaction of isocyanate groups with the substrate. Moreover, as thermoplastics, they are subject to both loss of adhesion and creep at high temperatures. U.S. Pat. No. 4,940,750 discloses a high shear method of preparation of such thermoplastics in an inert solvent.
U.S. Pat. No. 3,475,200 discloses a solventless method of preparing pastes useful for coating textile articles, containing a polyester diol or polyether diol, a uretdione diisocyanate and at least 0.3 mole of a solid chain extender having two hydroxyl or amino groups and a particle size less than 30 .mu.m. The compositions are said to have a shelf life of several days. U.S. Pat. No. 4,251,427 further refines such compositions by incorporation of aluminosilicate molecular sieves.
U.S. Pat. No. 4,400,497 discloses suspensions of solid isocyanates in a polyol wherein the isocyanate particles are deactivated on their surface such that from 0.5 to 20 percent of the total isocyanate groups are deactivated.
U.S. Pat. Nos. 3,488,302; 4,390,678; and 4,412,033 disclose one component urethane adhesives employing an isocyanate-terminated liquid prepolymer and an insoluble polyhydroxyl compound in solid form. The products are viscous liquids which heat cure at elevated temperatures.
U.S. Pat. No. 4,352,858 discloses compositions containing a liquid isocyanate-terminated prepolymer and molten dicarbamate esters which may be used for adhesives, optionally solvent borne.